Carbon material, particularly activated carbon material, is employed in a variety of fields; for example, it finds utility in treatment of water, catalyst carrier, gas occlusion and electric double layer capacitor electrodes. Among these, an electric double layer capacitor exhibits, for example, the following characteristics: rapid charging and discharging; resistance to excessive charging and discharging; long service life (since it does not undergo chemical reaction); a wide temperature range in which the capacitor can be used; and environmentally friendly nature (since it contains no heavy metal). Therefore, conventionally, an electric double layer capacitor has been employed in, for example, a memory backup power supply. In recent years, electric double layer capacitors of high capacitance have been developed rapidly, and such electric double layer capacitors have been employed in high-performance energy devices. Furthermore, an electric double layer capacitor has been envisaged to be employed in a power storage system in combination with a solar battery or a fuel cell, or to be employed for assisting a gasoline engine of a hybrid car.
An electric double layer capacitor includes a pair of positive and negative polarizable electrodes formed of, for example, activated carbon, the electrodes facing each other with the intervention of a separator in a solution containing electrolyte ions. When DC voltage is applied to the electrodes, anions contained in the solution migrate to the positively polarized electrode, and cations contained in the solution migrate to the negatively polarized electrode. Electric energy is obtained from an electric double layer formed at the interface between the solution and each of the electrodes.
Conventional electric double layer capacitors are excellent in power density but poor in energy density. Therefore, in order to realize employment of such an electric double layer capacitor in energy devices, capacitance of the capacitor must be increased further. In order to increase capacitance of an electric double layer capacitor, an electrode material which enables formation of a large number of electric double layers in an electrolytic solution must be developed.
An electrode predominantly containing activated carbon material is employed as a component constituting an electric double layer capacitor. Such activated carbon material is required to exhibit, as a key function, high capacitance per mass or per volume.
In view of the foregoing, use of an activated carbon material having a large specific surface area has been contemplated as an electrode material which enables formation of a large number of electric double layers. When such an activated carbon material is employed, capacitance per mass (F/g) increase, but capacitance per volume (F/ml) fails to increase to an intended level, because of lowering of electrode density.
In recent years, there has been proposed an approach to production of an activated carbon material containing microcrystals similar to those of graphite, along with employment of the thus-produced activated carbon material as a raw material for forming a polarizable electrode (Japanese Patent Application Laid-Open (kokai) No. 11-317333). In view that an electric double layer capacitor including a polarizable electrode formed from the activated carbon material exhibits high capacitance, the activated carbon material is considered an excellent electrode material.
However, the aforementioned activated carbon material is not necessarily satisfactory, in that it involves some problems. That is, since expansion of the activated carbon material occurs during application of voltage, as described in the above publication, a size-limiting structure must be provided for suppressing expansion of the activated carbon material, and thus difficulty is encountered in assembling a capacitor. In addition, application of a voltage of as high as about 4 V is required in advance in order to obtain sufficient capacitance of the capacitor. As a result, decomposition of an electrolytic solution may occur.
In a typical method for producing activated carbon material, an organic substance such as coconut shell, pitch, or phenol resin is thermally decomposed to thereby yield a carbonized material, and the carbide is activated.
Examples of activation methods include gas activation employing steam or carbon dioxide gas, and chemical activation employing, for example, potassium sulfide, zinc chloride, or an alkali hydroxide. Particularly, activation employing an alkali hydroxide such as potassium hydroxide or sodium hydroxide is effective for producing an activated carbon material having a large specific surface area, and an activated carbon material produced through this activation method exhibits high capacitance per mass or per volume.
When alkali activation, for example, activation employing an alkali hydroxide, is employed, an alkali metal compound is melted through heating, and a carbon material is impregnated and reacted with the molten alkali metal compound, to thereby form a porous structure and activate the carbon material. When alkali activation of powdery or granular carbon raw material is employed in a container such as a crucible, effervescence of a molten liquid occurs during activation, due to generation of, for example, moisture or hydrogen gas, and the molten liquid may overflow the container. Particularly when alkali activation is carried out at high temperature increase rate, the amount of gas generated in a unit time increases, and overflow of the molten liquid tends to occur. In order to avoid such overflow, the amounts of an alkali metal compound and a carbon raw material which are placed in a container must be limited, and therefore productivity of an alkali-activated carbon material is considerably lowered, resulting in high production cost.
Accordingly, an object of the present invention is to provide an activated carbon material which enables capacitance per electrode to be increased without application of a high voltage.
Another object of the present invention is to drastically improve the productivity in the activation of an activated carbon material and to produce an activated carbon material which is excellent in capacitance at high current density.